Finish bathroom / exterior wall not sealed

Hello, we have an extension. The exterior wall there needs to be resealed because we have over 70 percent humidity inside. Unfortunately, this will only happen at the end of September. Now I am wondering if it makes sense to at least start the bathroom (currently gladly being renovated) by applying liquid foil and tiling the floor... What do you think?

Isn't it better to start with the walls first? I watched our tiler, he also stuck this "sealing tape" at the corners between the wall and the floor... it's hard to see in the picture, but it's this dark strip.